1. Field of the Invention
The present invention relates to an electron source with emissive cathodes having microtips. It more particularly applies to the manufacture of cathodoluminescence-based display means excited by field effect emission and in particular to the manufacture of flat screens. It is also usable for the manufacture of electron guns or vacuum gauges.
2. Discussion of the Background
Microtip emissive cathode electron sources are already known from the following documents:
(1) FR-A-2593953 corresponding to EP-A-234989 and U.S. Pat. No. 4,857,161 PA0 (2) FR-A-2623013 corresponding to EP-A-316214 and U.S. Pat. No. 4,940,916 PA0 (3) FR-A-2663462 corresponding to EP-A-461990 and U.S. Pat. No. 5,194,780 PA0 (4) FR-A-2687839 corresponding to EP-A-558393 and to U.S. patent application Ser. No. 08/022,935 filed on Feb. 26, 1993 (Leroux et al).
Document (1) describes a process for the production of a cathodoluminescence-based display means excited by field effect emission, whose microtip electron source is formed on a glass substrate and has a matrix structure.
Documents (2), (3) and (4) describe improvements made to the source described in document (1). Documents (2) to (4) more particularly relate to an improvement to the emission uniformity by limiting the current in the microtips emitting most electrons.
This improvement is obtained by introducing an electrical resistor connected in series with the microtips. This resistor is formed from a resistive layer, which can be continuous or discontinuous.
FIG. 1 is a diagrammatic, partial view of a known microtip emissive cathode electron source described in detail in document (2). This known source has a matrix structure and an e.g. glass substrate 2, on which is optionally formed a thin silica film 4.
On said silica film 4, said source also has a plurality of electrodes 5 in the form of parallel conductive strips serving as cathode conductors and which constitute the columns of the matrix structure.
Each of the cathode conductors is covered by a resistive layer 7, which can be continuous or discontinuous (except at its ends in order to permit the connection of the cathode conductors to the polarizing means 20). An electrically insulating, silica layer 8 covers the resistive layers 7.
Above the insulating layer 8 are formed a plurality of electrodes 10, once again in the form of parallel conductive strips. These electrodes 10 are generally perpendicular to the electrodes 5 and serve as gates, which form the rows of the matrix structure. A resistive layer can optionally be placed above or below the electrodes 10.
In an improvement to the source known from document (2), at least one of the series of electrodes (cathode conductors or grids) is associated with a resistive layer and each electrode of said series has a lattice or mesh structure.
Thus, document (3) recommends the use of lattice-shaped cathode conductors in such a way that the microtips are located in the openings of the lattices of the cathode conductors.
In this configuration, the breakdown resistance of a microtip is no longer mainly dependent on the thickness of the resistive layer, but instead on the distance between the microtip and the corresponding cathode conductor.
Another improvement to microtip emissive cathode electron sources is provided by document (4). This improvement aims at reducing the short-circuiting risks between the rows and columns of the source. To do this, a maximum reduction takes place of the overlap areas between the two series of electrodes.
This is diagrammatically and partially illustrated by FIGS. 2 and 3.
FIG. 2 is a diagrammatic, partial plan view of an electron source described in document (4) and FIG. 3 a larger-scale, sectional view a long the axis III--III of FIG. 2.
This known, matrix structure source has an e.g. glass substrate 1 and optionally a thin silica film 6 on said substrate 1. On the silica film 6 is formed a series of parallel electrodes 3 serving as cathode conductors, each of the said electrodes having a lattice structure. They form the columns of the matrix structure.
These cathode conductors 3 are covered by a silicon resistive layer 9, which is itself covered by an electrically insulating, silica layer 11.
Above said insulating layer 11 is formed another series of parallel electrodes also having a perforated, but different structure, which is designed to minimize the overlap areas with the cathode conductors.
These electrons formed above the insulating layer 11 are generally perpendicular to the cathode conductors and constitute the grids 13 of the source. They form the rows of the matrix structure.
FIGS. 2 and 3 show a detail of one of the grids of this source known from document (4). This grid, carrying the general reference 13, has parallel tracks 14 orthogonally intersecting other parallel tracks 15. At the intersections of the tracks 14 and 15, the grid has widened areas 17, which are square here.
FIG. 2 shows that the overlap areas 16 of a cathode conductor 3 and the tracks 14 and 15 of the grid have a very small surface. The widened areas 17 are located in the center of the meshes of the lattice-shaped cathode conductor.
In the intersection areas of the cathode conductors and the grids, holes or more precisely microholes 18 are preferably formed in the thickness of the widened areas of the grid and in the thickness of the insulating layer 11. The microtips 19 of the source are located in these holes and rest on the resistive layer.
An assembly constituted by a microtip and a microhole forms an electron microemitter. The electron microemitters occupy the central regions of the meshes of the lattice of the cathode conductor, as well as the widened, square areas 17 of the grid.
The meshes of the lattice can have different shapes and different dimensions. For example, they can be square and have a side length of 25 microns. The number of holes and tips in each mesh can also vary. Thus, there can be e.g. 4.times.4=16 tips per mesh.
When the source described relative to FIGS. 2 and 3 is made to operate, a voltage is applied between the cathode conductor and the grid. This leads to a current, which passes through the resistive layer between the cathode conductor and the microtips.
The further the microtips from the cathode conductor, the greater the distance separating them and the higher the electrical resistance (due to the resistive layer) by means of which said microtips are connected to the cathode conductor and therefore the lower the current supplying said microtips.
FIG. 3 shows symbolically the electrical resistance r1 of the microtips located at the edge of the group of microtips corresponding to a mesh of the cathode conductor and the electrical resistance r2 of the microtips in the center of said group of microtips, r2 being greater than r1.
It results from what has been stated hereinbefore that in the mesh, the microtips located in the centre of the group and which are further removed from the cathode conductor than the microtips located at the edge of said group, emit less electrons than the latter.